Perhaps you nailed it when you mentioned gas pedal pressure... My wife primarily drives our 2015 and we both pretty much drive like an old lady and have never even tried the Sport Mode. I doubt ours has every been above 70 mph believe it or not. We also go according to service interval B and buy quality fuel, and also have never seen any DPF messages.My experience is very similar to yours with our 2015 Summit, my wife does mostly city driving with 2-5 mile trips, we've never seen the DPF message. I do go out of my way to buy diesel at a busy Shell station though, and I follow service interval B because of the frequent short trips.
Using good judgment of how much gas pedal pressure is enough to get you going until the turbo kicks should help reduce soot production, per articles like "Analysis of the Effect of Vehicle, Driving and Road Parameters on the Transient Performance and Emissions of a Turbocharged Truck"
I have only one question... shouldn't we be calling it the "diesel pedal" and not "gas pedal"?